1. Field of the Invention
The present invention generally relates to oil and gas well (borehole) logging tools, and more particularly to an improved method of measuring several different characteristics of a geologic formation, including resistivity, density and porosity, using a single borehole tool.
2. Description of the Related Art
Logging tools for measuring earth formation properties are well known, particularly those used in the location of underground petroleum products (oil and gas). Borehole logging instruments use various techniques to determine geophysical properties such as bulk density, porosity, water saturation, and hydrocarbon type and saturation. The lithology of the formation can also be predicted from wellbore instruments, i.e., whether the rock constituents are predominantly sandstone, limestone, dolomite, etc. From the measurement of these properties, the likelihood of producible quantities of hydrocarbons is calculated and optimized.
Techniques for ascertaining formation properties include those involving the use of radiant (electromagnetic) energy. For example, gamma rays are commonly used to measure bulk density of a formation by detecting such radiation as it passes through the formation, and relating the amount of detected radiation to the electron density of the formation. See, e.g., U.S. Pat. No. 4,297,575. Gamma rays can be emitted continuously from a source in the borehole tool and propagate outward into the formation. This approach is known as gamma-gamma logging, because gamma rays originate in the tool, and the backscattered rays are thereafter detected in the tool. A typical gamma-ray source is cesium-137. Formation properties can be determined based on the count rate or intensity of the gamma rays that are received at detectors located in the tool. Usually at least two detectors (far and near) are used, which allows a measure of formation density that is essentially independent of the mudcake surrounding the tool (the mudcake is the layer of solid material lining the open borehole that has consolidated from the drilling fluid).
Another common parameter which is measured in geophysical well log analyses is the formation photoelectric absorption cross-section. Photoelectric absorption (also known as the photoelectric factor, or Pe) is dependent on the average atomic number of the irradiated sample. The Pe cross-section refers to the profile of the photoelectric absorption of the formation along a borehole section which is being investigated. Quantitative methods have been devised in the prior art for measuring Pe. These measurements are useful in determining the formation lithology because of their sensitivity to, e.g., calcium.
One standard method for measuring Pe is used in the borehole tool sold by Schlumberger Technology Corp. under the trademark LDT. The LDT tool is a gamma-gamma device, and its method of operation is further described in U.S. Pat. No. 4,048,495. The determination of the photoelectric factor is accomplished by measurement of the shape of the detected gamma-ray spectrum. With a properly calibrated LDT, Pe can be inferred from the relationship between the count rates in a high energy window and a low energy window. A Pe measurement can be further utilized to determine absolute elemental concentrations, as disclosed in U.S. Pat. No. 4,810,876. See also U.S. Pat. No. 4,628,202 which sets forth a variation on the LDT methodology, by developing an interrelationship between the photoelectric factor and density.
Instead of providing a radioactive gamma-ray source, gamma radiation can be produced in the formation in response to a high-energy neutron source (i.e., a neutron accelerator located in the borehole tool). This technique is referred to as induced gamma-ray logging. The radiation is analyzed using one of two common techniques to determine the porosity (not density) of the formation. The two methodologies are referred to as GST (gamma spectroscopy tool) and C/O (carbon:oxygen). In the GST-type method, silica, calcium and hydrogen levels are broken out using spectral techniques, and the amount of hydrogen is compared to the combined amount of silica and calcium to determine porosity. See Fang et al., xe2x80x9cTransformation of Geochemical Log Data to Mineralogy Using Genetic Algorithms,xe2x80x9d Log Analyst, vol. 37, no. 2 (1996). In C/O systems, carbon and oxygen levels are determined using spectral techniques and the ratio is then related to porosity.
When the neutron source is pulsed, gamma rays are produced by one of three reactions: inelastic scattering of fast neutrons (neutrons with energies in the range of 0.1 to 14 MeV); thermal neutron capture (neutrons that have slowed to a thermal velocity of typically 0.025 eV); and delayed emission from isotopes formed by neutron activation. The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons are thermalized (slow down) and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters). See, e.g., U.S. Pat. No. 4,055,763.
Another standard measurement is the thermal neutron porosity. This measurement uses a chemical source of fast neutrons such as Am241Be or Ca252. The subsequent distribution of thermal neutrons is dependent on the hydrogen content of the incident media. This hydrogen content is then used to make porosity. A new system marketed as APS, accelerator porosity sonde, by Schlumberger, uses an accelerator to replace the chemical AmBe source. This system has an array of thermal and epithermal neutron detectors to measure the neutron spatial distribution to make a hydrogen-based porosity. See Scott et al., xe2x80x9cResponse of a Multidetector Pulsed Neutron Porosity Tool,xe2x80x9d SPWLA Logging Symposium (June 1994).
Oftentimes, in open-hole well-logging and logging-while-drilling (LWD), it is desirable to take a set of the foregoing measurements. One prior art tool 2, shown in FIG. 1, is referred to as a xe2x80x9ctriple combo,xe2x80x9d and measures resistivity (from an electromagnetic (EM) induction system 4), bulk density (from a system 6 which includes a photoelectric factor determination), and porosity (from a compensated neutron (CN) system 8). When combined with a natural gamma ray detector 10 and a spontaneous potential measurement from EM induction system 4, the measurement set provides a powerful basis for well log analyses to evaluate for oil and gas production. A sonic system may be added, which is referred to as a xe2x80x9cquad-combo.xe2x80x9d
While a triple-combo or quad-combo tool provides several useful measurements in one instrument, it still has certain disadvantages. First, the chemical sources (AmBe for the compensated neutron system and Cs for the density and Pe factor), present safety and health concerns to the well operator, as well as liability issues if the tool is lost or trapped in the well. The triple-combo tool is also particularly long, usually between 60 and 90 feet, making it more cumbersome, and requiring an extra length of xe2x80x9crat-holexe2x80x9d that must be drilled. These measurements also have a limited penetration, typically requiring pads and linkages to maintain proper borehole contact. It would, therefore, be desirable to devise a convenient method for measuring a set of parameters in an earth formation, which overcomes the foregoing limitations. It would be further advantageous if the method allowed other simultaneous measurements.
It is therefore one object of the present invention to provide an improved method of measuring a plurality of formation characteristics using a downhole tool.
It is another object of the present invention to provide such a method and tool which provides density, lithology and porosity measurements, but which uses a tool having a shorter length than conventional tools.
It is yet another object of the present invention to provide such a method and tool which uses a single accelerator-based sonde for a wide variety of traditional open-hole logging measurements.
The foregoing objects are achieved in a method for measuring characteristics of a geologic formation, generally comprising the steps of inducing gamma rays in the formation, detecting a gamma-ray spectrum associated with the gamma rays, measuring the photoelectric absorption (Pe) factor of the formation using spectroscopic measurements of the gamma-ray spectrum, calculating a neutron porosity of the formation using the spatial distribution of the induced neutrons, and determining a bulk density of the formation using the spatial distribution of the induced gamma rays. The measuring of the Pe factor may include the step of inferring the photoelectric absorption of the formation by directly mapping the spectroscopic measurements. The calculating of the porosity may include the step of relating the spectroscopic measurements to a hydrogen content of the formation. The determining of the density may include the step of computing a gamma diffusion length of the formation based on the spectroscopic measurements. In addition to these measurements, the invention contemplates the measurement of a resistivity of the formation, concurrently with said inducing step, using an electromagnetic induction system. The spontaneous potential of the formation may also be measured using the electromagnetic induction system. The gamma rays can be induced by pulsing a fast neutron source proximate the formation. A gamma-ray detector may be placed at a substantial distance from the source (e.g., eight feet) for measuring the natural radioactivity of the formation.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.